In P mode, obviously when taking photos in low light one's iso climbs to say 1600 or 3200 or whatever, my problem is that in these situations

Exactly what are the situations? IOW if you are outdoors and using the pop-up @ 30' you probably can expect it to kick the iso up as far as it can, but in normal situations where the pop-up can provide any benefit at all, the iso will not go to its highest. How/where are you using the pop-up to add light?

when I pop up the flash the iso still reads 1600 or 3200 or whatever, surely with the flash lighting the scene when the photo is taken the the iso shoot drop way down, but it doesn't, so I find myself each time when using flash having to first go and take auto iso off (to avoid high iso's in low light conditions) and then dial in a lower iso, so tedious though.

When my D300 meters and shows me an iso of 3200 w/o flash (dark room lit by TV only and distance of 18ft.), and then I pop up the flash, the indicated iso drops to an iso of 200. When I shoot, the actual/final iso may end up being 350 or 450 or so, depending upon distance. It never holds to an iso of 1600 or 3200 or anywhere near that level.

I would have thought the camera would know that using the flash in the shot to come would not necessitate such a high iso... or am I missing something? or does the cam adjust the iso just prior to the shot?

It appears that is what my D300 does.

Which camera do you have?

Regards, Jim

cos in the reading on the screen is 1600 or 3200 or whatever but that's before it fires the shot (and flash)...maybe the 3200 is cos that's the light measurement prior to flash but as the flash goes off maybe the iso then drops for the shot itself.. any insight would be appreciated thanks.